1. Field of the Invention
The invention relates to a semiconductor laser, and particularly, to a semiconductor laser which allows a laser beam emitted therefrom to be collimated thus changing the characteristics of the beam.
2. Description of the Related Art
The invention relates to a semiconductor laser, and particularly, to a semiconductor laser which allows a laser beam emitted therefrom to be collimated thus changing the characteristics of the beam.
A semiconductor laser is widely utilized for communications such as optical communications and displays such as laser displays because of the wavelenuth monochromaticity thereof. The characteristics of a laser beam required for communications and displays are extremely differentiated, and it is difficult to satisfy all of the requirements in such various application fields using a typical single semiconductor laser chip. For example, in an actual application of DWDM (dense wavelength division multiplexing) requiring very high monochromaticity of a laser beam, the monochromaticity should have a high SMSR (side mode suppression ratio) of −30 decibel (dB) or more, the wavelength should be very stable, and numerous wavelengths at intervals of 200 GHz, 100 GHz, 50 GHz, 25 GHz, etc. should be used.
Because it is difficult to achieve the requirements in such communication networks using only the single semiconductor chip, additional parts having various functions may be integrated with a semiconductor laser diode chip, for example, a thermo electric cooler for controlling the temperature of the laser diode chip may be installed to one side of the semiconductor laser diode chip, or a wavelength-selective filter able to adjust the oscillation wavelength of the laser diode chip may be provided, thereby satisfying various requirements in communication networks. In particular, methods have been proposed to control the oscillation wavelength of a laser for communication in the form of an external resonator. As such, the external resonator type laser indicates that the feedback of the beam and the laser operation do not occur in the gain medium of the laser but the feedback of the beam takes place in a region different from the gain medium.
A typical single semiconductor chip laser causes the feedback of a laser beam using both ends of the gain medium as a reflective mirror, whereas an external resonator type semiconductor laser is configured such that at least one side of the semiconductor laser is subjected to non-reflective treatment and the feedback of the beam occurs outside the semiconductor gain medium. Thus, various optical devices are disposed between the semiconductor gain medium and the external reflection mirror, thereby manufacturing lasers having a variety of novel characteristics which cannot be obtained when using the typical single semiconductor chip.
FIG. 1 shows the structure of an external resonator type semiconductor laser, published by N. K. Dutta et al. (JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-21, NO. 6, JUNE 1985). As shown in FIG. 1, this laser diode chip is of an edge emitting type which emits light from one side of the laser diode chip and is configured such that the optical axis of the laser diode chip and the optical axis of the lens coincide with each other. Typically, the edge emitting type laser diode chip is assembled so that the wide bottom surface of the chip is attached to the bottom of a package and thus the optical axis of the lens becomes parallel to the bottom surface. Only in the case where the beam emitted from the laser diode chip is fed back to the laser diode chip after collimation using the lens and then determination of the wavelength of the beam by means of a wavelength-selective filter, it becomes possible to control the wavelength using the external resonator as desired in the cited reference of FIG. 1.
FIG. 2 shows the travel path of the beam when the optical axis of a laser diode chip 100, the optical axis of a lens 200 and the optical axis of a final reflection mirror 500 coincide with each other. As used herein, the final reflection mirror 500 indicates a reflection mirror that functions to return a laser beam by a path reverse to a path in which a laser beam enters the reflection mirror at an incident angle normal to the surface of the reflection mirror. As shown in FIG. 2, the laser beam emitted from the laser diode chip 100 at a wide angle is collimated into a parallel beam through the lens 200, and the collimated beam is exactly perpendicularly reflected from the final reflection mirror 500, and thereby the laser beam is fed back to the laser diode chip 100 along the travel path reverse to the travel path from the laser diode chip 100 to the final reflection mirror.
FIG. 3 shows the travel path of the beam under conditions in which the optical axis directions of the laser diode chip 100 and the final reflection mirror 500 coincide with each other, the optical axis direction of the lends 200 coincides with the laser diode chip 100 and the final reflection mirror 500 but the center line of the optical axis thereof is spaced apart from the center line of the optical axis of the laser diode chip 100. When the optical axis of the laser diode chip 100 is parallelly spaced apart from the optical axis of the lens 200, the optical axis of the laser beam passed through the lens 200 is inclined to the optical axis of the laser diode chip 100. Thus, the laser beam is not perpendicularly incident on the final reflection mirror 500, whereby the laser beam reflected from the final reflection mirror 500 is not fed back to the laser diode chip 100.
FIG. 4 shows the travel path of the beam when the optical axis of the laser diode chip 100 and the optical axis of the lens 200 coincide with each other and do not coincide with the optical axis of the final reflection mirror 500. Also in this case, it is obvious that the beam not be fed back to the laser diode chip 100.
FIG. 5 shows the travel path of the beam under conditions in which the optical axis of the final reflection mirror 500 is inclined to the optical axis of the laser diode chip 100 and the optical axis of the lens 200 is spaced apart from the optical axis of the laser diode chip 100 so that the laser beam passed through the lens is perpendicularly incident on the final reflection mirror 500. The beam which is perpendicularly incident on the final reflection mirror 500 is perpendicularly reflected therefrom. In this case, even when the optical axis of the final reflection mirror 500 does not coincide with the optical axis of the laser diode chip 100, the laser beam emitted from the laser diode chip 100 is reflected from the final reflection mirror 500 and is thus fed back to the laser diode chip 100. That is, in the case of a reflection mirror which is planar, changes in the position of the reflection mirror that does not change the inclined angle to the incident optical axis result in no change in the travel angle of a beam. However, in the case of a lens, when the central optical axis direction of the lens is fixed and the position of the optical axis is changed, the travel angle of a beam may be changed. In the optical system comprising the laser diode chip, the lens and the final reflection mirror, it is difficult to very precisely fix the positions of respective three parts. Hence, these parts are typically configured such that the laser diode chip and the final reflection mirror are first fixed, and the position of the lens is then precisely controlled so that the laser beam emitted from the laser diode chip is perpendicularly incident on the reflection mirror and thus the laser beam reflected from the reflection mirror is fed back to the laser diode chip. For the sake of brief description, the reflection mirror may be used, but this reflection mirror may be manufactured to further include an additional optical element such as a wavelength-selective filter able to select the wavelength of a laser beam or a wavelength-variable filter able to alter the selected wavelength.
FIG. 6 shows the structure of a typical edge emitting type semiconductor laser diode chip 100. The typical semiconductor laser diode chip 100 has a cuboid structure in which an active region where a laser beam is produced is provided on the upper portion of the chip 101 and the height of the chip 100 is shorter than the width and length of the chip 100. Such a chip 100 is installed so that the active region is parallel to the bottom plane 810 of a package including the laser diode chip 100 as shown in FIG. 7, in order to effectively remove heat generated from the laser diode chip 100.
In the case where the laser diode chip 100 is disposed parallel to the bottom plane 810 of the package and the final reflection mirror 500 is disposed perpendicular to the bottom plane of the package, as described in FIG. 2, the position of the lens 200 should be controlled on the x-z plane of FIG. 7 so that the optical axis of the laser beam emitted from the laser diode chip 100 and the optical axis of the final reflection mirror 500, which do not coincide with each other, are made to coincide with each other. Thus, the position of the lens should be controlled in the form of being placed in the air with respect to the bottom plane 810 of the package, and special elements for fixing the position-controlled lens 200 at the controlled position should be further added. In order to move the lens 200 on the x-z plane, the height of the lens 200 cannot be fixed, and thus the lens 200 should be undesirably continuously gripped until the lens 200 is arranged and fixedly attached to additional lens supports which are not shown in the drawing.
In the cited reference of FIG. 1 using the wavelength-selective filter which is of a reflection type, the external resonator type laser is employed in which the wavelength-selective filter itself functions as a reflection mirror, but this method is not well utilized because it is difficult to control the characteristics of the wavelength-selective filter. Separately, there are devised methods in which the wavelength-selective filter is manufactured in the form of a transmission type and an additional reflection mirror is used, and these methods facilitate the wavelength-selective filter to be imparted with various characteristics and are thus mainly adopted in the external resonator type laser.
FIG. 8 shows the structure of an external resonator type laser which uses a transmission wavelength-selective filter able to alter the selected wavelength and is of a wavelength-variable type able to alter the wavelength selected by the wavelength-selective filter. (U.S. Pat. No. 7,295,582 B2 M. E. McDonald et al.,) in the description of FIG. 5, the lens is mentioned in which its height is fixed. However, in the case where the laser diode chip, the lens and the reflection mirror are fixed at predetermined positions, the optical axis of the laser beam travelling to the reflection mirror through the lens must exactly coincide with the optical axis of the reflection minor at a very high precision so that the beam passed through an Etalone filter is perpendicularly incident on the reflection mirror and perpendicularly reflected therefrom.
FIG. 9 shows the results of arrangement of the laser diode chip 100, the lens 200 and the final reflection mirror 500 in the structure of FIG. 7, as tested in the laboratory. FIG. 9 shows a laser spectrum when the laser beam travelling to the final reflection mirror 500 through the lens 200 of FIG. 7 very highly coincides with the optical axis of the final reflection mirror 500, and FIG. 10 shows a laser spectrum when the lens is horizontally moved by about 0.5 μm all in the x direction of FIG. 7 with the optical axis direction of the lens 200 being maintained under conditions of FIG. 9. In this case, the laser diode chip 100 and the final reflection mirror 500 are pre-fixed and thus the optical axes thereof are not changed, and the optical axis direction of the laser beam which reaches the final reflection mirror 500 through the lens 200 becomes slightly distorted due to the movement of the lens 200.
The spectrum characteristics of two types of FIGS. 9 and 10 are remarkably different, which means that the ratios of the beam reflected from the final reflection mirror and fed back to the laser diode chip 100 differ considerably from each other. Accordingly, the lens 200 should be very precisely arranged so that the laser beam incident on the final reflection mirror 200 enters the final reflection mirror 500 at a very exact normal angle. So, the arrangement precision of the lens 200 on the x-z plane in FIG. 4 should be preferably less than +/−0.5 μm under optimal conditions, and more preferably less than +/−0.1 tall under optimal conditions.
In FIG. 7, the sensitivity of position precision of the lens 200 in the y axis direction is remarkably lower compared to the x axis and y axis directions, and thus may be about 5 μm. Specifically, the arrangement precision of the lens should be preferably less than 0.1 μm with respect to the plane perpendicular to the optical axis, but allows about 5 μm in the optical axis direction of the lens. Thus, precise arranging becomes possible in such a manner that the lens, the position of which was preset in the optical axis direction, is manufactured and then moved on the perpendicular plane of the optical axis and thus may be precisely arranged. The comparatively high position precision of about 0.5 to or 0.1 to is very difficult to achieve using the process of individually assembling the laser diode chip, the lens and the final reflection mirror. So, there is typically employed a method in which a laser diode chip and a final reflection mirror are preset and then a lens is arranged at a position where a laser beam emitted from the laser diode chip is reflected from the final reflection mirror and is the most effectively fed back to the laser diode chip. Accordingly, In FIG. 5, the height of the lens should be fixed after very precisely arranging the lens, and thus a special element for precisely arranging and fixing the lens on the x-z plane is required.
An aspect of the invention is to provide a method in which the lens 200 is arranged/attached on the x-y plane instead of being arranged/attached on the x-z plane as seen in FIG. 7, and thereby the position of the lens in contact with the bottom plane of a package is controlled, so that the optical axis of the laser beam incident on the final reflection mirror and the optical axis of the final reflection mirror coincide with each other. In the invention, the height of the lens is an optical axis direction of a laser beam, and thus there is no need for a very high precision of about 0.1 μm, and the height of the lens may be previously fixed by being attached to the preset lens supports. Accordingly, the lens supports function to support the lens thus ensuring the stable position, thereby eliminating the need to continuously grip the lens during assembling the external resonator type laser and arranging the lens. Furthermore, a higher degree of freedom may be imparted in fabricating the lens supports for holding the lens and fixing the lens supports having the lens attached thereto on the bottom plane of a package.
FIG. 8 shows an external resonator type semiconductor laser in which a wavelength-variable Etalone filter able to control the wavelength of the transmitted beam is interposed between the semiconductor gain medium and the reflection mirror. In the external resonator type laser, a semiconductor laser having various characteristics may be manufactured depending on the characteristics of the optical device interposed between the gain medium and the reflection minor. Another aspect of the invention is to provide various embodiments of the external resonator type laser which is easily constructed using the idea of the application.